The ability to determine the identity of a chemical entity in a complex mixture has a broad range of highly useful applications. The techniques traditionally used in analysis of complex mixtures include chromatography and mass spectrometry. Although both chromatography and mass spectrometry separate a complex mixture into constituent parts, neither technique provides direct identification of the chemical constituents. Rather, the identity of a chemical constituent must be determined based on an analysis of the measured characteristics of the chemical constituent.
As used herein, the term “identification” as applied to chemical entities refers to the high confidence determination of the identity of a chemical entity. An example of identification is the determination that a molecule having 7 carbon atoms, 7 hydrogen atoms, a nitrogen atom, and 2 oxygen atoms is anthranilic acid rather than salicylamide, both of which have the same chemical formula C7H7NO2.
This ability to perform non-targeted analysis, such as initial detection and subsequent recognition of unknown metabolites, has enormous benefits. For example, in a metabolic analysis of cells with and without cancer, if the analysis results show that cancerous cells almost always contain a certain unknown molecule while healthy cells do not; these results give important direction to research for detection or treatment of that cancer.
Metabolomics includes the ability to perform non-targeted analysis, which means that a chemical constituent may be detected and subsequently recognized, even though it may not be identified.
Currently, methods exist to determine the elemental compositions of ions in a mass spectrum. This knowledge greatly reduces the number of possible compounds that could produce a particular mass spectrum. One can conclusively refute as candidate compounds those that provide similar low resolution mass spectra containing a molecular ion or a fragment ion with a different ion composition. Review of the chemical and commercial literature can further limit the probable identity of an analyte to one or a few compounds. However, in many cases the number of compounds with the same composition is large or the chemical classes of such compounds may represent multiple chemical classes. Thus, even when the list of candidates is reduced to only a few compounds, confirmation is time and resource intensive. In many cases the standards for possible candidates cannot be purchased and instead must be synthesized de novo which can be expensive and time consuming.
Therefore a need exists to improve the ability to elucidate the identity of an unknown compound by narrowing the list of candidate compounds to chemicals from the same biochemical class (e.g., amino acids, fatty acids, carbohydrates) and to further limit the candidates within a particular class.